David Cameron has denied claims he is scared of TV election debates, saying he wanted to "get on" with his proposal for a seven-way contest.
Ed Miliband has accused the PM of "cowering" from the public after he rejected proposals for a head-to-head debate with the Labour leader.
But the prime minister claimed the broadcasters were to blame.
Meanwhile, the BBC Trust has rejected the Democratic Unionist Party's appeal against its exclusion from the debates.
The decision is likely to trigger a judicial review by the DUP.
'Name the date'
Explaining his decision to reject the broadcasters' debates proposals, Mr Cameron said that rather than ducking scrutiny, he wanted to "unblock the logjam" the "broadcasters helped to create".
"Let's get on, let's have the debate that matters the most," added Mr Cameron. By putting forward a proposal for a debate with seven leaders, "we'll actually see one take place", he said.
Mr Miliband said he did not accept the prime minister's excuse and accused him of "running scared" from TV debates.
"I'll debate him, any time, any place, anywhere. He should stop ducking and weaving and he should name the date," said the Labour leader.
"I think what the public will not tolerate is a prime minister who is running away from them (the debates), running away from his record and running away from a face-to-face debate with me that he said he wanted and the public deserve."
Lib Dem election campaign chief Lord Ashdown said Mr Cameron was "running scared" and was "frightened of defending his own position".
"What he is proposing is not just a ludicrous, seven-sided, bite-sized squabble fest but actually he is proposing it takes place before the Conservative manifesto is published," he told BBC Radio 4's Today programme.
Mr Clegg said: "If David Cameron is too busy or too important to defend the record of this government with Ed Miliband then I offer myself. How about that? I'll do it instead."
He told LBC radio the Conservative leadership was behaving like "they're ordering a drink in the drawing room of Downton Abbey and, sort of, telling everybody else what they should do". He said: "I mean it's not for one party to, you know, grandly tell everybody else what's going to happen."
Under Mr Cameron's "final offer", one 90-minute contest would take place before 30 March, when the official general election campaign starts.
Downing Street said the Democratic Unionist Party should also be considered for inclusion.
The broadcasters, who have proposed a total of three debates, said they would respond to the Conservatives' proposal in due course.
The BBC and ITV are scheduled to hold two, both involving the Conservatives, Labour, Liberal Democrats, UKIP, Plaid Cymru, the SNP and the Greens.
A third debate - hosted by Sky and Channel 4 - the week before the 7 May election would feature a head-to-head between Mr Cameron and Mr Miliband.
After Mr Cameron questioned the timing of it at Prime Minister's Questions on Wednesday, Sky and Channel 4 offered to move the event to a different date if the leaders could agree.
Analysis by BBC assistant political editor Norman Smith
So are the debates dead?
Well, maybe not. But only if the broadcasters hold their nerve.
In other words, if they decide to press ahead with the three debates and empty chair the prime minister.
It would be a huge decision - and many at Westminster remain sceptical that the BBC would be willing to do this.
However, privately, the broadcasters insist they will not buckle and will not allow one party to "dictate" the conditions.
They insist the single, 90-minute, seven, or even eight party, debate proposed by the prime minister will "not cover the ground".
And crucially, both Labour and the Liberal Democrats say they will still turn up for whatever debates the broadcasters decide to hold.
Ed Miliband will even take part in the head-to-head without David Cameron - and subject himself to a grilling from Jeremy Paxman.
Senior Lib Dems say Nick Clegg would be ready to stand in for the prime minister in the final head-to-head, making it a Miliband v Clegg clash.
The danger for the prime minister is that even if the debates lose their impact without him - he risks a backlash from voters for failing to take part.
Downing Street's hope is that the broadcasters will buckle and either agree to his proposal or just scrap the whole idea of TV debates for this election.
A UKIP spokesman said: "After praising what a good thing debates were for democracy as recently as 2014, why is David Cameron now acting chicken and running as far away from them as possible?"
UKIP leader Nigel Farage said Mr Cameron had "sabotaged the whole thing" with his proposal, which would not be a "proper debate".
SNP leader Nicola Sturgeon accused Mr Cameron of "arrogance," adding: "I will debate him anytime, anywhere, on any number of occasions.
"However we have accepted the broadcasters' proposals, and believe we should stick with that, rather than allow a Tory prime minister to dictate the terms of debate."
The Green Party has accused Mr Cameron of "further damaging trust" in British politics.
"Not only is Cameron's announcement cowardly but it also shows his contempt for the electorate. People want to see a set of debates between all major party leaders, yet the prime minister is clearly scared of scrutiny," said a spokesman.
Plaid Cymru leader Leanne Wood said Mr Cameron's "efforts to manipulate the broadcasters are unacceptable and arrogant".
Speaking after his party's appeal to be included was rejected, Nigel Dodds, the DUP's Westminster leader, said: "I think that the decision of the BBC Trust defies belief, quite frankly, because it's wrong, irrational, and unjust to exclude Northern Ireland from the national debates if you are going to include the Scottish National Party from Scotland and Plaid Cymru from Wales."
As leader of the opposition, Mr Cameron led calls for televised debates at the 2010 general election - the first time they have been held in the UK.
But he has since criticised the idea of holding debates in the run up to polling day, saying they had "sucked the life out of the campaign".
After months of wrangling over the format for debates at the 2015 election, Downing Street issued a statement on Wednesday night, confirming that Mr Cameron did not want to go head-to-head with Labour leader Ed Miliband under any circumstances.
In a letter to the broadcasters, the prime minister's communications chief Craig Oliver said: "In order to cut through this chaotic situation I am willing to make the following proposal: There should be one 90-minute debate between seven party leaders before the short campaign."
He added: "In order for it to be organised in time, the debate should take place during the week beginning March 23. I will make myself available to negotiate the details. Having been the editor of numerous broadcast news and current affairs programmes, I know this is ample time to organise a programme.
"This is our final offer, and to be clear, given the fact this has been a deeply unsatisfactory process and we are within a month of the short campaign, the prime minister will not be participating in more than one debate."
In a joint statement, broadcasters said: "The BBC, ITV, Channel 4 and Sky have received an email from the prime minister's office with a proposal.
"The broadcasters are committed to providing our audiences with election debates.
"Twenty two million people watched the debates in 2010 and we believe the debates helped people to engage with the election.
"The broadcasters have set out their proposals and continue to talk to all the relevant parties on an equitable basis.
"We will respond to the Conservatives' proposal in due course."